Exoplanets

One of the most exciting astronomical developments in the past 15 years has been the discovery and charac­terization of extrasolar planets—planets in orbit around stars other than our Sun, a.k.a. “exoplanets.” Direct detection of gas-giant exoplanets is now becoming possible, opening the door to new avenues of understanding distinct from radial velocity and transit detections.

Habitable Extrasolar Planets.

Nader Haghighipour studies potentially habitable extrasolar planets — ones that have temperatures and surface gravities conducive to the presence of liquid water. During the past five years, he has been involved in a program to detect these objects, both on theoretical and observational fronts and since 2009 has has been searching for small planets using the HIRES spectrometer on the Keck I telescope. He also collaborated with the UC Santa Cruz Lick Observatory, and with Carnegie Institute of Washington to use their telescope in Las Campanas, Chile. During the past three years, his project has resulted in the detection of 12 planets among which are two Earth-sized planets in the habitable zone, namely Gliese 581g, the very first habitable planet de­tected, and GJ 667Cc, a recently identified habitable planet.

Artistic rendering of the habitable planet GJ 667Cc around the M star GJ667C

Hunting for other worlds

Michael Liu is currently completing an unprecedented 250-star high-contrast imaging campaign to detect and characterize young (less than one billion years) extrasolar planets and brown dwarfs using the Near-Infrared Coronographic Im­ager (NICI) on the Gemini South 8.1 m telescope.

NICI is the first instrument designed from the outset for high-contrast imaging on a large telescope. It comprises a high-performance adaptive optics system with a simultaneous dual-channel coronographic imager. Funded by NASA, NICI was built with substantial involvement from the IfA, including faculty members Mark Chun and Christ Ftaclas.

The NICI Campaign achieves about 2 magnitudes better contrast compared with any previous ground-based or space-based planet-finding efforts inside of ~2 arcsec separations. To date, the NICI Campaign has discovered three new substellar (~30 Jupiter mass) companions to young stars, as well as several dozen exoplanet candidates that are in the process of being confirmed/refuted through high-precision, second-epoch astrometry.

PZ Tel B is a substellar companion to the brown dwarf star PZ Tel A. The vast majority of light from PZ Tel A has been removed from this image using specialized image analysis techniques. The size of the orbit of Neptune is shown for comparison.

High-Resolution Submillimeter Imaging of a Planet-Forming Disk

Graduate student Geoff. Mathews, supervised by Jonathan Williams, used the Submillimeter Array to study a protoplanetary transition disk in the nearby Upper Scorpius region at 0.3'' resolution. The disk is seen to be nearly face on and has a clear central cavity about 70 AU in radius. The existence of a central, dust-depleted hole had been inferred from the pronounced dip in the spectral energy distribution, but the SMA observations allow them to directly measure the size of the gap, determine the dust surface density profile and characterize the sharpness of the inner disk edge. A detailed truncated accretion disk model that simultaneously fits the spectral energy distribution and SMA spatial information is shown to fit the data very well. The “ringing” in the radially averaged visibility profile is an indication that the edge is quite sharp which, in turn, suggests that the inner regions have been dynamically carved out by one or more protoplanets rather than by other proposed processes such as grain growth, which produce more gradually tapered disk holes. The disk is also gas rich, and we are carrying out further work to understand the chemistry and implications for planet formation.